1. Field of the Invention
The present invention relates to a circuit for sequentially turning on a plurality of power units which supply power to respective devices such as central processing unit and magnetic tape unit.
Such a system as consists of a plurality of devices, each of which has a respective power unit, is being adopted in recent years. Therefore, an economical and reliable power system of this type is increasingly being demanded.
2. Description of the Related Art
In the above-mentioned power system, if all the power units are turned on at a time, a rush (or surge) current 10 to 20 times as large as the steady-state current flows, damaging external power source circuit or requiring as large current capacity as the rush current of the external power source. Therefore, a method to sequentially turn on the power units as shown in FIG. 1 and in Laid Opened Japanese Patent TOKUKAISHO 59-123425 (1984) is being used.
FIG. 1 is a block diagram illustrating a sequential power-on method of the related art of the present invention.
Power unit 1a is placed first in line and followed by power units 2a to 5a, the order in which power is turned on. External power source is connected in common to terminals L1-L2. Power units 1a to 5a each have power supply circuit 12 for supplying power to the devices (not depicted via terminals T1-T2. Power supply circuit 12 of power unit 1a is directly connected to terminals L1-L2, and those of power units 2a to 5a are connected thereto via, for example, relay contacts which are closed by a start signal from the preceding power unit of the line. The start signal, which commands that the succeeding power unit be turned on, is activated when a power supply circuit is turned on.
When turned on, the external power source is input directly to power supply circuit 12 of power unit 1a, which supplies power to the device and outputs a start signal to power unit 2a via terminal D1.
In power source unit 2a, the start signal received at terminal Al closes the contacts to input the external power supply to power source circuit 12. Thus, power unit 2a is turned on and a start signal is output to power unit 3a.
In a similar manner, subsequent power units 3a to 5a are turned on in sequence.
However, a problem is that in case the start signal is not propagated to the next power unit due to a failure in any of the power units, all the subsequent power units cannot be turned on, disabling the system.